Underground drilling, such as gas, oil, or geothermal drilling, generally involves drilling a bore through a formation deep in the earth. Such bores are formed by connecting a drill bit to long sections of pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface to the bottom of the bore.
The drill bit is rotated so that the drill bit advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string at the surface. Piston-operated pumps on the surface pump high-pressure fluid, referred to as “drilling mud,” through an internal passage in the drill string and out through the drill bit. The drilling mud lubricates the drill bit, and flushes cuttings from the path of the drill bit. In the case of motor drilling, the flowing mud also powers a drilling motor, commonly referred to as a “mud motor,” which turns the bit, whether or not the drill string is rotating. The mud motor is equipped with a rotor that generates a torque in response to the passage of the drilling mud therethrough. The rotor is coupled to the drill bit so that the torque is transferred to the drill bit, causing the drill bit to rotate. The drilling mud then flows to the surface through an annular passage formed between the drill string and the surface of the bore.
Typically, measurements are taken of various operating parameters during drilling. For example, surface equipment senses the rate of penetration of the drill bit into the formation, the rotational speed of the drill string, the hook load, surface torque, and pressure. Sensors either at the surface or in a bottom hole assembly, or both, measure the axial tensile/compression load, torque and bending. However, selecting the values of the drilling parameters that will result in optimum drilling is a difficult task. For example, although reducing the downhole force applied to the drill bit, commonly referred to as the “weight on bit” (“WOB”) or the rotary speed of the drill bit may reduce vibration, and thereby extend the life of drill string components, it may also reduce the rate of penetration (“ROP”). In general, optimal drilling is obtained when the rate of penetration of the drill bit into the formation is as high as possible while the vibration is as low as possible. The ROP is a function of a number of variables, including the rotational speed of the drill bit and the WOB.
Techniques have been developed to estimate the energy expended to drill through a fixed volume of rock—in other words, the ratio of the energy input into the drilling to the output of the drilling in terms of ROP—which is referred to as the Specific Energy. One measure of the Specific Energy is the Mechanical Specific Energy (“MSE”), which is a measure of the mechanical energy required to drill through a fixed volume of formation, obtained by determining the ratio of the rate of the mechanical energy usage to the ROP. More recently, another measure of the specific energy, referred to as the Hydro Mechanical Specific Energy (“HMSE”) has been developed to take into account the hydraulic, as well as the mechanical, energy expended during drilling. Attempts have been made in the prior art to utilize the specific energy, especially the MSE, to optimize drilling performance by favoring operation at conditions that will result in a low value of MSE. However, depending on the characteristics of the drilling operation, operating a minimum value of MSE does not uniformly result in maximizing drilling performance. Therefore, an ongoing need therefore exists for methods of optimizing drilling performance and monitoring the drilling performance on an on-going basis to determine whether drilling conditions have changed, warranting further optimization.